#980
phil
Forum Participant

A variac is good to bring the power up slowly but the rectifier itself won’t usually pass current until you get up to about 80 or 90 volts.

Anyway… some suggestions :

A variac is useful for powering on slowly, or for running a radio at lower power , eg 110 instead of 120V. The old analogy is that if powered on slowly it will reform the electrolytics, but many collectors today will say any electrolytic you find today in an old radio should just be replaced for safety and reliability. – So wheather or not reforming or replacing is "reasonable" is subjective, and opinions do vary.

An isolation transformer is reccomended for bench safety especially when working with transformerless AC radios. an isolation transformer has no connectivity between the primary and secondary – most variacs do, thus are not isolation transformers.

#1 dim bulb tester. Here is a cheap first project that is highly recommended.
Just take a short extension cord cut one wire, and hook a light bulb socket across the cut leads. cut the wire from the thinner prong if it is a polarized plug. Presto you have one !

you can add a 1 amp or so fuse and a switch and a ammeter too if you like. you can add a red "on light" ( across the two wires) after the switch just to have as an indicator as to weather you have the power turned on to the device. . you can switch both wires for safety with a double pole switch if you want to.

You can use different light bulbs to vary the output voltage, but most importantly if you plug in something that is shorted the light bulb will burn full brightness .You won’t blow your house breaker, instead you will promptly switch it off and investigate the reason for high resistance.
As a test plug in a radio that works to see how it reacts.

Be ready to unplug your new project radio should you see the bulb at full brightness. The filter capacitors might be shorted. Sometimes the power transformers can be shorted but not directly shorted. if it is shorted half way through the windings it can run without blowing thebreaker but the power transformer will get too hot. that will make the bulb burn more brightly than normal.

I like to replace the filter capacitors and any that show obvious signs of aging before any power is applied. Check the radio over for any scary connections, rubber wire that is now gush..blackened resistors, etc. fix that first. Then slowly power on and watch observe what gets hot. increase voltage either with the variac or by swapping light bulbs, until I am "brave" enough to plug it in.

Then I unplug and replace one or two caps at a time, just to doublecheck that I haven’t made a mistake. hear it come to life as I go.

"tip" when changing capacitors, clip a couple of test clips onto where it came from, and mark it on the schematic. that way if the phone rings..
take some pictures as you go !

Meter – you probably have one of some sort. nice to have a digital meter and an analog, A vacuum tube volt meter is a good thing to look for as well.

Signal generator , you need one to allign the radio. They usually come at reasonable prices if you wait and watch. Most will cover BC band. For now, if you don’t have one you could download one these ones off the web. for example :
https://heliso.tripod.com/download/generator/dsg.htm
( search for signal generator download if you don’t want this particular one , there are others , even ones with a scope as well)

*These computer signal generators put the signal out through your sound card. I made a "dummy antenna" and found if I hold it near the antenna it will put out the signal just fine. I know I won’t blow my computer if it isn’t connected. You would need to be careful if you actually connect it to the radio. I tested it by beating the signal against my signal geerator and found it to be very accurate. I gave my wife and myself a hearing test by going to higher frequencies She can hear higher notes than I can 🙂

Soldering gun , just a typical weller gun works fine for me, a soldering station would be nice.
I have a massive pencil type iron that is great for soldering to a thick chassis, and a small pencil iron for anything too delicate for the gun.

Solder sucker- needed, a cheap one with a spring loaded trigger is all I have I am sure others have nicer ones.

Signal tracer – will "listen" to the signal , not needed. fun to play with. I bet you could make one out of a radio.

Resistors , nice to have a little assorment of 2 watt resistors. you can often check them as you change the caps so having a few handy is nice. a small variety of higher value 5-10 W resistors is good to have as well, but you can always buy them as you need them.

table. try to find a spot where you can solder and it won’t bug anyone. I like to have a project going and return to it at will. cleaning up after each session would be difficult. some prefer to stand , and others like to sit when working. It is handy if you can see your computer from your bench to be able to research , read schematics etc , but again – a preference, A computer can be a distraction too.

I hope this helps, Hopefully others will offer opinions on what is most needed, and their techniques for a basic powering on, recapping, allignment etc. I would be interested to know what equipment others most often reach for.

Phil